1. Field of the Invention
This invention relates to a spark plug electrode in which a spark-erosion resistant noble metal containing tip is secured to a firing end of a center or ground electrode, and particularly to an improved spark plug electrode in which a noble metal component of the tip is altered depending on a total magnitude of the spark discharges to which a specified portion of the tip is subjected.
2. Description of Prior Art
With the recent demand of enhancing a spark-erosion resistant property of an electrode for use in an internal combustion engine, Japanese Patent Publication No. 62-31797 introduces a technique in which annular noble metal tips 103, 104 in turn cover firing ends of center electrode metals 101, 102 of multi-polarity type and semi-creeping discharge type spark plugs 100, 100a as shown in FIGS. 11 and
Due to the positional relationship between the center electrode metals 101, 102 and ground electrode 105, 106, the tips 103, 104 tend to be locally eroded in a manner as shown at a left half in FIGS. 11 and 12. The spark erosion makes spark gaps 107, 108 unacceptably greater to exhaust service life of the spark plug, and an expensive noble metal component of the tip partially remains uneroded so as to make the technique uneconomical.
In order to avoid the uneconomical disadvantage, it is considered to previously leave out an uneroded part of the noble metal tip as shown at the left half 109 in FIG. 13a.
However, the center electrode metal 101 is spark eroded significantly as spark discharges repeatedly occur between the electrodes as shown in FIGS. 13b through 13d. This makes the service life of the spark plug 110 increasingly shorter than that of the spark plugs 100, 100a.
This is because the spark discharges presumably occur to the portion 109 of the electrode 101 in which the noble metal tip 103 is left out although a total magnitude of spark discharges is substantially limited when compared to the portion to which the noble metal tip 103 is subjected.
Therefore, it is an object of the invention to provide a spark plug electrode for use in an internal combustion engine which is capable of controlling a rapid spark erosion of an electrode to which a limited magnitude of spark discharges is subjected while maintaining an economical advantage by substantially reducing a quantity of noble metal component used for a portion of the electrode to which the limited magnitude of spark discharges is subjected.